This invention relates to computed x-ray tomography and is particularly concerned with providing gamma correction to the digital pixel values that compose an x-ray image.
In computed tomography, the image pixels are represented by CT numbers. CT numbers are proportional to the x-ray attenuation by small volume elements in the body being scanned with an x-ray beam. In a sense, CT numbers represent the brightness of the pixels which compose an image. Although CT numbers correspond to x-ray intensities or brightness, any output device by which the image is displayed whether it be film, a television monitor and even the eye itself is non-linear. By way of example, a pixel value or CT number of 50 may not appear on the television screen as being twice as bright as an input CT number of 25. Thus, the CT number or digital pixel data is usually modified by a correction factor which is commonly called gamma correction. Gamma corrections differ for direct viewing of an image on a TV screen and for filming an image from a TV screen. In the latter case, for example, the non-linear responses of the television monitor and film must be taken into account.
The multicell detectors that are used for sensing x-ray attenuation by the body in computed tomography apparatus and for providing intensity signals to a computer are extremely sensitive to small differences in x-ray intensity. Using data provided by the detector cells, the computer algorithm operates to produce a range of CT numbers with greater x-ray intensity resolution or gray scale steps than can be distinguished by the human eye. For example, present tomographic image reconstruction methods are capable of quantitizing x-ray intensity measurements into 4096 separate intensity levels. Human viewers, on the other hand, are typically incapable of discerning more than approximately 64 visually discernable shades or gray scale levels. Hence, it has been the practice to select from a large range of CT values a limited range called a window and to display pixels having CT values within the window over the gray scale capability of the television tube in which case CT numbers above the upper window limit are white and those below the lower window limit are black.
It has been customary in computed tomography systems to provide the user with controls for selecting window level (L) and window width (W) and for selecting the gamma correction table that is appropriate for the manner in which the image is displayed or viewed. Note that window is defined as the window white value minus the black value (WBV). The number of CT numbers within the window is the window width plus one. The window width and level functions are used to expand or compress the gray scale around a given CT number. With a wider window, more CT numbers are displayed. Narrower windows display fewer CT numbers. By way of example, narrow windows are used to investigate tissue such as in the brain where small variations in density must be discerned to distinguish healthy from unhealthy tissue. Wide windows may be used to investigate bony structures where there are wide variations of density.
Heretofore, two different methods were used for making the appropriate gamma correction in relation to the level and window which had been selected. One method uses a dedicated microprocessor to supply the window value and level value to one window-level look-up table and gamma data to another gamma correction look-up table. Each table had a particular transfer function. The array of digital pixel values or CT numbers representing the image were held in a full-frame image refresh memory. Before a video image could be provided, the window-level look-up table and the gamma look-up table had to be loaded. Typically, this took about 200 ms. However, it is customary for the user to adjust the window level while viewing the image on the television tube. During adjustment, a scrambled or blanked image appeared on the screen. This was distracting to the observer.
In the other prior art method, a microprocessor is used to load a window-level look-up table and the data therein is multiplied by a suitable factor to provide gamma correction. This process may be somewhat faster but it still has the disadvantage of scrambling or blanking the image on the display tube during window and level adjustments.